We have seen that there are several processors by which material degrades in the environment.
Whereas metals are prone to oxidation by the oxygen in air, and plastics, fibreglass, rubber, clothes and other materials are prone to oxidation by UV light, by far the most common mechanism of degradation is biodegradation, specifically of organic material.
Stated simply, this explains how things rot. When food goes off in the fridge, it is biodegradation. When you see the carcass of a rotting kangaroo by the side of the road, that is biodegradation.
There are two types of biodegradation – aerobic and anaerobic.
As the name suggests, aerobic degradation occurs when there is oxygen present, and anaerobic degradation occurs when there isn’t.
The way it works is this – microorganisms (commonly referred to as simply bugs) convert large organic (carbon-based) molecules into carbon dioxide. This is an aerobic process because they require the oxygen to make the carbon dioxide. This is why if you accidentally leave a carton of milk on the table for a few days, it is all bloated when you discover it. The expansion is caused by the CO2 that is generated by the degradation process.
This is an extremely important process which is widely used in the industrial sector. The most common place is in waste treatment plants, both domestic and industrial.
By domestic, we are simply talking about sewerage plants. In these plants, using a process known as the activated sludge process, naturally occurring bacteria convert human waste, and other domestic waste into CO2, nitrogen, sulphates, and biomass. The role of the plant operator is to manage the plant such that there is sufficient oxygen available for the bacteria to do their job. If you were to visit one of these plants you would typically see large blowers blowing air into the water like a giant aquarium bubbler in your fish tank. This is simply to provide enough oxygen for the aerobic bacteria to do the job.
This biodegradation is also extremely important industrial process, and has become more so in recent years. Now, when an industry wishes to process the waste from an industrial process, the use of biological plants (often in conjunction with chemical plants) is very common. These of course are used widely mostly in areas where the waste is organic in nature – food processing and so on.
it is also used to regenerate contaminated groundwater or soil. Where an industrial pollutant has leaked into groundwater or soil, often the best way to degrade it is the naturally occurring bacteria in the soil – all they need is enough oxygen to do the job. A common approach at these sites, therefore, is to drill holes in the ground and insert huge air blowers which just blast area into the ground. The air allows the bacteria to do their job (and their population increases as a result and the process becomes more efficient) and often all but the most recalcitrant chemicals (which will look at tomorrow) can be removed. It is, for example, a common process where petroleum solvents are the culprit.
So what sort of chemicals can be biodegrade, and which ones can’t?
In general, any naturally occurring chemical will be prone to biodegradation – this includes even crude oil as we have seen before. The danger, of course, is that synthetic materials are not prone to biodegradation readily. Plastic bags are a good example – the polymer chains are extremely stable and resist the action of bacteria. But they are not so much of a problem, as they are chemically inert – the worst to say about them is that they are unsightly if they cause litter – but there are no toxicity issues.
Of much greater concern, are synthetic chemicals, which were made (and put into the environment) in an age where people just didn’t care about whether they were biodegradable or not. They were made, and used, because they were cheap, they did the job, and they were stable – that is they didn’t degrade this time. So the very thing that made them attractive in the first place, now is causing a problem.
This is a large topic in itself, and targets chemicals in several different industries – and we will look at it tomorrow.